This invention relates to the problem of determining unknown constituents and/or measuring constituent concentrations in a fluid, under circumstances such that a "contaminant" or extraneous constituent of the fluid interferes with monitoring of the constituent sought. If the contaminant concentration fluctuates, accurate subtraction of the background due to the contaminant is difficult. Resort to dual-channel monitoring systems (that is, addition of a "reference" monitor) can solve the problem only if it is possible to provide a second fluid stream which contains substantially just the contaminant -- and the contaminant therein must be at concentrations which accurately track the contaminant concentrations in the sample fluid.
A case in point is the analysis of a gas sample by gas chromatography. Such measurements can be particularly valuable when using an infrared spectrometer as a detector, because qualitative as well as quantitative analysis is possible. In such instrumentation, one of the significant limitations upon measurement accuracy is error due to contamination of the sample by gaseous material which "bleeds" from the coating on the adsorbing surface in the chromatographic column. Such bleeding of the column coating material is especially troublesome in "temperature-programmed" gas chromatography, wherein the rising column temperature tends to volatilize or decompose the adsorbing material, yielding constituents in the effluent which are unrelated to the test sample being analyzed. In such a case it is not obvious how to obtain a signal related purely to the bled coating material; consequently it has in prior devices not been possible to obtain saisfactory correction for or cancellation of the bled coating.
Some workers have attempted to solve this problem by providing a second chromatographic column, not injected with the sample gas, to provide suitable background gas for the reference cell of the spectrometer. This technique requires that the two columns be closely matched as to the coated adsorbing layer, on either the column wall or its packing, and as to carrier-gas flow rate as well as temperature, throughout their respective lengths. An accurate balance is not easily maintained.
Similar difficulties can arise in the case of a chromatograph operated with a carrier gas which has a constituent (such as, for example, either a contaminant, or the carrier gas itself) which interferes with measurement of the sample gas.
Analogous unsatisfactory background-matching attempts are familiar in other fluid-stream-measurement situations.
In measurements employing spectrally-distinguished characteristics, regardless of whether chromatographs are used, it is possible for the spectral effect of an extraneous constituent of even very weak concentration to interfere with interpretation of the spectrum of a desired constituent -- because the extraneous constituent may have relatively intense spectral features.
My invention resolves all these difficulties for situations in which the contaminant concentration in a fluid stream changes slowly compared with the sample-constituent concentration(s) of interest.
My aforementioned U.S. Pat. No. 3,897,154 describes one solution to the problem described above, which solution involves directing the fluid stream through both measuring stations of a dual-channel monitoring system. The following ten paragraphs are devoted to describing the general concept of my invention as described in my aforementioned patent. It should be emphasized that these paragraphs are for reference only and do not describe the specific aspects of my invention to which the claims of this application are addressed.